Heat the oil in a large skillet over medium-high heat. Add the onions and cook, stirring occasionally, until browned, about 7 minutes. Add the chickpeas, raisins, harissa, cinnamon, cumin, 1/2 teaspoon salt and 1/4 teaspoon pepper and cook, stirring, until the spices are toasted, about 30 seconds.
Working over the skillet, rip the tomatoes into large chunks with your hands and add to the skillet; add 2 cups water. Reduce the heat to medium-low, cover and simmer until the tomatoes and chickpeas have softened, about 12 minutes. Stir in the squash and olives, cover and cook until the squash is tender, about 5 minutes. Stir gently, so as not to break apart the squash too much, and simmer, uncovered, until the sauce is slightly thickened, about 5 minutes.
While the tagine simmers, prepare the couscous according to package directions.
Remove the tagine from the heat, and stir in the cilantro. Adjust the seasoning with additional salt, pepper and harissa. Divide the couscous between four plates, and spoon the tagine over it. Serve with additional harissa on the side.
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